1. Field of the Invention
The present invention relates to an electronic copying machine having a developing device which uses a toner, for development, and more particularly, to a method for controlling the developing device in a toner-empty state.
2. Description of the Related Art
Electromagnetic copying machines are often provided with a developing device which develops an electrostatic latent image on a photoconductor by means of the magnetic brush method.
In order to keep the copy density of resulting images constant, in copying machines of this type, the density of the toner in the developing agent in the developing device is detected by means of a density sensor, and the detection signal is compared with a preset reference value. A toner supply unit is used to replenish the developing device with toner as required. By doing this, the toner density in the developing agent can be kept constant.
If the toner in the toner supply unit is reduced below a predetermined amount in copying machines constructed in this manner, a toner-empty state is indicated, and an instruction is given for toner supply.
In some conventional copying machines, the copying operation is prohibited when a toner-empty state is encountered. In others, the copying operation can be continued for repeated copying cycles even in the toner-empty state.
Immediately after a toner-empty state is encountered, the toner density in the developing agent is not too low, so that several cycles of normal copying operations can be executed. This cannot be done, however, according to the former control method in which the copying operation is prevented at the time when the toner-empty state is detected. In particular, if the toner-empty state is encountered in the course of continuous copying, the copying operation is interrupted despite the possibility of the several cycles of normal copying operation, which is a wholly situation.
Meanwhile, the copying operation can be unlimitedly performed according to the latter control method in which the operation is allowed to be continued even after the occurrence or on-set of a toner-empty state. Initially, therefore, several cycles of normal copying operations can be carried out. If the toner density falls drastically, however, images will be developed only by means of the carrier.